The present application relates to lift cranes, and particularly to mobile lift cranes having a counterweight that can be moved to different positions in an effort to balance the combined boom and load moment on the crane.
Lift cranes typically include counterweights to help balance the crane when the crane lowers its boom and/or lifts a load. Sometimes the counterweight on the rear of the crane is so large that the carbody is also equipped with counterweight to prevent backward tipping when no load is being lifted. Further, an extra counterweight attachment, such as a counterweight trailer, is sometimes added to the crane to further enhance the lift capacities of the mobile lift crane. Since the load is often moved in and out with respect to the center of rotation of the crane, and thus generates different moments throughout a crane pick, move and set operation, it is advantageous if the counterweight, including any extra counterweight attachments, can also be moved forward and backward with respect to the center of rotation of the crane. In this way a smaller amount of counterweight can be utilized than would be necessary if the counterweight had to be kept at a fixed distance.
A typical example of the forgoing is a Terex Demag CC8800 crane with a Superlift attachment. This crane includes 100 metric tonne of carbody counterweight, 280 metric tonne of upperworks counterweight, and 640 metric tonne on an extra counterweight attachment, for a total of 1020 metric tonne of counterweight. The extra counterweight can be moved in and out by a telescoping member. While all of this counterweight makes it possible to lift heavy loads, the counterweight has to be transported whenever the crane is dismantled for moving to a new job site. With U.S. highway constraints, it takes 15 trucks to transport 300 metric tonne of counterweight.
Since the crane needs to be mobile, any extra counterweight attachments also need to be mobile. However, when there is no load on the hook, it is customary to support these extra counterweights on the ground apart from the main crane; otherwise the extra counterweight would generate such a moment that the crane would tip backward. Thus, if the crane needs to move without a load on the hook, the extra counterweight attachment also has to be able to travel over the ground. This means that the ground has to be prepared and cleared, and often timbers put in place, for swing or travel of the extra counterweight unit. Thus, there is a benefit to a crane design that has movable counterweight that does not need to be supported by the ground except through the crawlers on the crane.
U.S. Pat. No. 7,546,928 discloses several embodiments of mobile lift cranes with a variable position counterweight that have high capacities with lower amounts of counterweight, and the movable counterweight does not need to be supported by the ground. While these embodiments are great improvements in the high-capacity crane design, there are cranes with lower capacities for which it would also be desirable to increase the capacity of the crane without increasing the total counterweight of the crane, especially if the counterweight did not need to be supported by the ground during crane operation. Further, the cranes in the '928 patent include a fixed position lattice mast structure from which the counterweight is suspended by a tension member. Sometimes it is beneficial if the mobile lift crane does not have a fixed mast structure, since the lattice mast structure requires additional components to be delivered to a job site, and a high fixed mast is sometimes an obstacle requiring clearance when the crane is repositioned. Thus, there is a need for further improvements in counterweight systems for mobile lift cranes.